Spring Boot has become the go-to framework for Java developers when building microservices and web applications. Its ease of use, rapid development features, and production-ready capabilities make it a popular choice among enterprises. As a result, Spring Boot interviews are common for Java development roles.

In this comprehensive guide, we’ll explore 40+ frequently asked Spring Boot interview questions and answers, covering beginner to advanced levels. Whether you’re a fresher or an experienced professional, these questions will prepare you to crack your next Spring Boot interview with confidence.

Table of contant

What is Spring Boot?

Answer:
S.B is an open-source framework developed by Pivotal Team. It simplifies the development of stand-alone, production-grade Spring-based applications. It minimizes the configuration required by offering default setups, auto-configuration, and opinionated defaults.

1. What are the key features of S.B?

Answer:

• Auto-Configuration: Automatically configures Spring applications based on dependencies.
• Standalone Applications: Runs independently using embedded servers like Tomcat, Jetty, or Undertow.
• Opinionated Defaults: Provides default configurations to reduce boilerplate code.
• Production-Ready: Integrates health checks, metrics, and monitoring via Spring Boot Actuator.
• No Code Generation: Uses runtime configurations instead of code generation.
• Microservices Support: Best suited for developing microservice-based applications.
• For a detailed comparison of Spring Boot and Spring Framework, visit our S.B vs Spring Framework page.

2. How is Spring Boot different from the Spring framework?

Comprehensive Guide to Spring Boot Interview Questions

Answer:

3. What is S.B Starter?

Answer:
A S.B Starter is a set of convenient dependency descriptors that aggregate commonly used libraries. Starters simplify dependency management by providing a one-line entry in the Maven or Gradle build file.

Example:

xmlCopyEdit<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-web</artifactId>
</dependency>

4. What is S.B Auto-Configuration?

Answer:
S.B Auto-Configuration automatically configures Spring application components based on the dependencies present in the classpath. This helps developers focus on writing business logic instead of configuration code.

Annotation:

javaCopyEdit@EnableAutoConfiguration

5. Explain Spring Boot annotations.

Answer:

• @SpringBootApplication: Combines @Configuration, @EnableAutoConfiguration, and @ComponentScan.
• @RestController: A combination of @Controller and @ResponseBody for REST APIs.
• @RequestMapping: Maps HTTP requests to handler methods.
• @GetMapping, @PostMapping: Specializations of @RequestMapping.
• @Entity: Specifies that a class is an entity and is mapped to a database table.

6. What is the purpose of the application.properties file in S.B?

Answer:
application.properties is the default property file used to configure Spring Boot applications. It contains key-value pairs for configuring the application’s behavior.

Example:

iniCopyEditserver.port=8081
spring.datasource.url=jdbc:mysql://localhost:3306/mydb

Answer:

• application.properties: Uses key-value pairs.
• application.yml: Uses hierarchical structure with indentation, making it more readable for complex configurations.

Example of application.yml:

yamlCopyEditserver:
  port: 8081

8. What is S.B Actuator?

Answer:
Spring Boot Actuator provides built-in endpoints for monitoring and managing a Spring Boot application in production.

Common endpoints:

• /actuator/health: Displays health information.
• /actuator/info: Shows application info.
• /actuator/metrics: Provides performance metrics.

9. How can you change the default server port in S.B?

Answer:

Modify application.properties:

iniCopyEditserver.port=9090

Modify application.yml:

yamlCopyEditserver:
  port: 9090

10. What are embedded servers in Spring Boot?

Answer:
Spring Boot includes embedded servers like:

• Tomcat (default)
• Jetty
• Undertow

These servers allow Spring Boot applications to run as standalone Java applications without external deployment.

11. How do you create a S.B project?

Answer:

• Use Spring Initializr: https://start.spring.io
• Select dependencies, project type (Maven/Gradle), and packaging (Jar/War).
• Download the project and import it into your IDE.

12. What is the default scope of beans inS.B?

Answer:
The default scope is Singleton, which means a single instance of the bean is created and shared throughout the application.

13. What is the use of @SpringBootApplication?

Answer:
@SpringBootApplication is a convenience annotation combining:

• @Configuration
• @EnableAutoConfiguration
• @ComponentScan

It marks the main class of a Spring Boot application.

14. How do you handle exceptions in S.B REST API?

Answer:

• Using @ControllerAdvice and @ExceptionHandler annotations.

javaCopyEdit@ControllerAdvice
public class GlobalExceptionHandler {
  @ExceptionHandler(Exception.class)
  public ResponseEntity<String> handleAllExceptions(Exception ex) {
    return new ResponseEntity<>(ex.getMessage(), HttpStatus.INTERNAL_SERVER_ERROR);
  }
}

15. How do you configure a database in S.B?

Answer:

• Add database dependency (e.g., MySQL).
• Configure properties in application.properties:

iniCopyEditspring.datasource.url=jdbc:mysql://localhost:3306/mydb
spring.datasource.username=root
spring.datasource.password=pass
spring.jpa.hibernate.ddl-auto=update

16. What is Spring Data JPA?

Answer:
Spring Data JPA simplifies database access using repository interfaces. It reduces boilerplate code by providing built-in methods like findAll(), save(), deleteById(), etc.

17. How to create a REST controller in S.B?

Answer:

javaCopyEdit@RestController
@RequestMapping("/api")
public class UserController {
  @GetMapping("/users")
  public List<User> getAllUsers() {
    return userService.getAllUsers();
  }
}

18. What is the difference between @Component, @Service, and @Repository?

Answer:

• @Component: Generic stereotype for any Spring-managed component.
• @Service: Specialization of @Component for service layer.
• @Repository: Specialization for DAO layer, with automatic exception translation.

19. How do you enable scheduling in S.B?

Answer:

• Use @EnableScheduling in the main class.
• Annotate methods with @Scheduled.

javaCopyEdit@Scheduled(fixedRate = 5000)
public void scheduleTask() {
  System.out.println("Running scheduled task");
}

20. How do you secure a S.B application?

Answer:

• Add Spring Security dependency.
• Configure security rules in SecurityConfig class.

javaCopyEdit@EnableWebSecurity
public class SecurityConfig extends WebSecurityConfigurerAdapter {
  @Override
  protected void configure(HttpSecurity http) throws Exception {
    http.authorizeRequests().anyRequest().authenticated().and().httpBasic();
  }
}

21. What is the purpose of S.B DevTools?

Answer:
Spring Boot DevTools accelerates application development by enabling hot reloading, automatic restart, and enhanced debugging support.

22. What are Profiles in S.B?

Answer:
Spring Profiles provide a way to segregate application configuration for different environments (dev, test, prod).

Define profile-specific properties:

• application-dev.properties
• application-prod.properties

Activate profile:

iniCopyEditspring.profiles.active=dev

23. How do you implement pagination and sorting in S.Bt?

Answer:

javaCopyEditPageable pageable = PageRequest.of(page, size, Sort.by("name"));
Page<User> users = userRepository.findAll(pageable);

24. What is a CommandLineRunner in S.B?

Answer:
CommandLineRunner runs specific code once the application context is loaded.

javaCopyEdit@Bean
public CommandLineRunner demo(UserRepository repository) {
  return (args) -> {
    repository.save(new User("John", "Doe"));
  };
}

25. How does S.B achieve loose coupling?

Answer:
Spring Boot uses Dependency Injection (DI) to promote loose coupling between objects, making the code modular and easier to maintain.

26. How do you create custom exceptions inS.B?

Answer:

javaCopyEditpublic class ResourceNotFoundException extends RuntimeException {
  public ResourceNotFoundException(String message) {
    super(message);
  }
}

Handle with @ControllerAdvice:

javaCopyEdit@ExceptionHandler(ResourceNotFoundException.class)
public ResponseEntity<String> handleResourceNotFound(ResourceNotFoundException ex) {
  return new ResponseEntity<>(ex.getMessage(), HttpStatus.NOT_FOUND);
}

27. What is spring-boot-starter-test?

Answer:
It is a starter for testing Spring Boot applications, including libraries like JUnit, Mockito, and AssertJ.

28. How do you deploy a Spring Boot application?

Answer:

• Package as a JAR:

bashCopyEditmvn clean package
java -jar target/myapp.jar

• Deploy WAR to external servers (Tomcat).

29. What is a Bean in Spring Boot?

Answer:
A Bean is an object that is managed by the Spring IoC container. Beans are created and wired automatically based on configurations.

30. How do you override default properties in Spring Boot?

Answer:

• Use application.properties.
• Set properties as command-line arguments:

luaCopyEditjava -jar app.jar --server.port=9090

• Environment variables.

31. What is Hystrix in Spring Boot?

Answer:
Hystrix is a fault tolerance library from Netflix, integrated with Spring Boot to manage latency and failures in microservices.

32. What is the difference between JAR and WAR in Spring Boot?

Answer:

33. How do you consume REST APIs in Spring Boot?

Answer:

• Use RestTemplate or WebClient:

javaCopyEditRestTemplate restTemplate = new RestTemplate();
ResponseEntity<String> response = restTemplate.getForEntity("https://api.example.com/users", String.class);

34. What is Feign Client in Spring Boot?

Answer:
Feign Client is a declarative web service client. It simplifies REST API calls between microservices.

javaCopyEdit@FeignClient(name = "user-service")
public interface UserClient {
  @GetMapping("/users/{id}")
  User getUserById(@PathVariable("id") Long id);
}

35. How do you enable CORS in Spring Boot?

Answer:

javaCopyEdit@Bean
public WebMvcConfigurer corsConfigurer() {
  return new WebMvcConfigurer() {
    @Override
    public void addCorsMappings(CorsRegistry registry) {
      registry.addMapping("/**").allowedOrigins("*");
    }
  };
}

36. What are DTOs in Spring Boot?

Answer:
DTO (Data Transfer Object) is a design pattern to transfer data between processes, reducing unnecessary data exposure.

37. What is the difference between @RestController and @Controller?

Answer:

38. How do you use properties in Spring Boot classes?

Answer:

javaCopyEdit@Value("${server.port}")
private int port;

Or use @ConfigurationProperties for mapping properties to POJOs.

39. How do you implement logging in Spring Boot?

Answer:

• Use built-in SLF4J with Logback.

javaCopyEditprivate static final Logger logger = LoggerFactory.getLogger(MyClass.class);
logger.info("This is an info message");

40. What is the purpose of @EnableAutoConfiguration?

Answer:
@EnableAutoConfiguration tells Spring Boot to automatically configure beans based on the classpath settings and defined beans.

Conclusion

This list of 40+ Spring Boot interview questions and answers covers the fundamental and advanced concepts you need to succeed in your next interview. Whether you’re preparing for a role in microservices architecture, REST API development, or backend systems, mastering these questions will give you a solid edge.

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